The present invention relates to television color subcarrier generators, and more particularly, to one for the PAL (phase alternating line) standard.
In the PAL color standard, the phase of the color subcarrier (as measured by its initial zero axis crossing) relative to the horizontal sync pulse repeats every 8 fields (4 frames). Further the frequency of the subcarrier is an odd multiple plus either one (PAL-N) or three (PAL-B) quarters of the horizontal frequency. In addition, there is an offset of 25 Hz, which is one half vertical frequency, added to the color subcarrier frequency to produce motion of the color dot pattern that occurs upon display, thereby reducing its visibility. During certain operations, such as tape editing, it is important that the phase relationship be held within tight tolerances to avoid picture disturbances, see "E. B. U. Review Technical Report" No. 172. In particular, the E. B. U. proposes that the tolerance be .+-.2.5.degree..
A typical prior art circuit for generating the PAL subcarrier signal having the 25 Hz frequency offset uses mixers, phase shifters, and frequency dividers. In general, due to aging and temperature drifts, the above components may introduce phase errors into the subcarrier signal (f.sub.sc) relative to horizontal sync greater than said tolerances.
It is therefore desirable to provide a color subcarrier generator having a high degree of phase accuracy.